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Full text of "Alaskan glacier studies of the National Geographic Society in the Yakutat Bay, Prince William Sound and lower Copper River regions"

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LUCIA AND ATREVIDA GLACIERS                            7S
debris-covered ice. The surface of the white area was crevassed as badly as that of the surrounding debris-covered glacier. The appearance of this area of white ice (pp. 78-9) in the midst of moraine-covered ice, and where debris had previously covered the glacier, was a puzzling feature. The significance of this feature is discussed in another chapter (p. 187), after other similar areas have been described.
The view of the broken glacier from Terrace Point (PL XIX, A) showed clearly that in 1906 Atrevida Glacier was impassable from as far up the mountain valley as we could see down to the outer border of the crevassed area in the alder zone. This view, žover a month later than the one from Amphitheatre Knob, showed considerable change in detail through ablation, but otherwise no notable difference in condition. The western margin of the glacier near Terrace Point consisted of a high, crevassed, inaccessible ice precipice replacing the moderately-sloping, moraine-covered embankment, up and down which we easily traveled in 1905. By cutting ice steps all the way this margin could be ascended; but further progress was barred by a complex network of profound crevasses, with intervening ridges and pinnacles, that could not be crossed with a camping outfit. It was a wonderful contrast to the easily-traversed surface of 1890 and 1905.
Along the eastern margin titanic changes, quite like those along the margin of the advancing Marvine Glacier were in progress before our very eyes. At Esker Stream the ice front had advanced an unknown amount, destroying the vegetation on the slopes of Amphitheatre Knob. The moderately sloping cliff of 1890 and 1905 was changed to a jagged precipice of broken ice blocks, down which debris was incessantly falling. The cave from which Esker Stream issued had disappeared through ice faulting (PI. XXI, A), and the stream had increased in volume. The faulted, splintered ice cliff extended westward with overhanging cornices due to horizontal thrust faulting (PI. XVII, B). In the one place where the cliff could be ascended, further advance was halted by deep crevasses before the level of the glacier surface was reached. The morainic soil, with the bushes growing upon it had disappeared into crevasses or had slid down the front of the glacier. Moraine, ice, and wood were constantly sliding down from the glacier front, the avalanches sometimes including score of tons, and it was always dangerous to the observer. By this material the coniferous forest growing in front of Atrevida Glacier was being tilted, buried, and destroyed, including alders, underbrush, and trees over 50 years old (PI. XTm", B). The breaking also resulted in exposing to melting the buried ice that had long been protected by moraine and forest, so that augmented streams were rapidly cutting into the soil, gravels and rock. All this change had taken place between August 23,1905 and June 26,1906.
Condition in 1909. Knowing of no previous observations upon the behavior of glaciers, undergoing abrupt change from long continued stagnation to great activity, and from unbroken surface to impassably crevassed condition, we could not, in 1906, predict with any certainty what was in store for the Atrevida Glacier. In view of the suddenness of the transformation, and the rapidity with which its full effects spread over the glacier,, we were prepared to believe that the cycle of activity would be a brief one if the cause of earthquake impulse were the correct explanation. We did not, however, suspect what the evidence of the 1909 observations clearly demonstrate, that the advance had passed its height and had almost ceased when the Atrevida Glacier was being studied in the summer of 1906. The probable future of Atrevida Glacier predicted in 1906 by the senior author was that the alder and forest growth within the crevassed area would